The present invention relates to a lockup control apparatus and method for a vehicle drive train control system.
The vehicle drive train includes an engine, a transmission having an input shaft and an output shaft, a torque converter, between the engine and the transmission, including a lockup clutch hydraulically engaged to various degrees for establishing various torque transmitting capacities in transmitting torque between the engine and the input shaft of the transmission in response to a control signal, and a control unit having stored therein data, prepared for keeping the lockup clutch engaged during coasting of the vehicle, for developing the control signal in response to the stored data during coasting of the vehicle.
For improved fuel economy, it is desired to keep the lockup clutch engaged during coasting of the vehicle without occurrence of engine stall. Theoretically, if torque transmitting capacity of the lockup clutch is lower or smaller than a lower limit value, there occurs considerable amount of slip in the look-up clutch, while, if torque transmitting capacity of the look-up clutch is higher than or greater than an upper limit value, there occurs an unacceptable delay in response time from a moment when a look-up release command is issued to a moment when disengagement of the look-up clutch is completed. Thus, it is desired to keep torque transmitting capacity within a predetermined window or range defined between or by the lower and upper limit values. This window should be narrow and extends in the neighborhood of the lower limit value for quick response of the lockup clutch to the lockup release command during coasting of the vehicle.
During coasting of the vehicle, the input shaft of the transmission drives the engine, while, during driving of the vehicle, the engine drives the input shaft of the transmission. The torque converter with the lockup clutch is well known. One example is disclosed in pages A-14 to A-15 and A-43 to A-45 of a publication entitled "Service Manual For Nissan Full-range Electronically Controlled Automatic Transmission 5E-AT, RE5R01A Type" published by Nissan Motor Co., Ltd. in June 1989. This known torque converter comprises a converter cover drivingly connected to the engine, a pump impeller connected to the converter cover, a turbine runner situated between the converter cover and the pump impeller, a stator situated between the pump impeller and turbine runner, and a lockup piston connected to the turbine runner for rotation therewith. The turbine runner is drivingly connected to the input shaft of the transmission. During release of the lockup clutch, a hydraulic medium, namely a converter activating pressurized hydraulic fluid, is supplied from a lockup control valve to a first passage opening to a space between the converter cover and the lockup piston, separating a clutch facing of the lockup piston from the converter cover. The hydraulic medium supplied to this space flows through spaces between the pump impeller, turbine runner and stator and flows out of the torque converter through a second passage opening into the pump impeller. Under this condition, the lockup clutch is disengaged or engagement of the lockup clutch is released. To establish engagement of the lockup clutch, a flow of hydraulic medium is changed. Hydraulic medium is supplied to the second passage and the first passage is connected to a drainage, allowing discharge of hydraulic medium from the space between the cover and the lockup piston. Rate of discharge of hydraulic medium is affected by centrifugal force imparted to the hydraulic medium within the torque converter. With the structure of the converter, speed of rotation the lockup piston does not exceed speed of rotation of the converter cover during driving of the vehicle where the engine drives the vehicle load. However, during coasting of the vehicle where the engine is driven by the vehicle load, speed of rotation of the lockup piston is higher than speed of rotation of the converter cover unless the lockup clutch is engaged. The lockup control valve is operatively coupled with a lockup control means or element in the form of a lockup solenoid. The duty of the lockup solenoid is adjusted in response to a control signal indicative of a desired duty. The control signal is developed by a control unit. For control of torque transmitting capacity of the lockup clutch to keep engagement of the lockup clutch during coasting operation, if the stored data of the control unit is modified in a direction to cause an increase in torque transmitting capacity of the lockup clutch each time after it is determined that there is occurrence of slip during coasting of the vehicle, actual torque transmitting capacity tends to increase toward or beyond the above-mentioned upper limit value. Thus, this control strategy is not satisfactory.
An object of the present invention is to provide a lock-up control strategy for control of torque transmitting capacity of the lockup clutch of the torque converter during coasting of the vehicle wherein torque transmitting capacity of the look-up clutch is adjusted to keep an improved response of the lockup clutch to a lockup release command with good fuel economy maintained during coasting of the vehicle.